Apparatus for detecting the level of an object surface

ABSTRACT

An apparatus for detecting the level of an object surface comprises a light source for supplying a light beam obliquely incident on the object surface, an imaging optical system provided to image the light beam reflected by the object surface, a light-receiving element having a detecting surface coincident with the imaging plane of the imaging optical system and determining the level of the object surface on the basis of the position of the image of the light beam on the detecting surface, and polarization correcting optics provided on an optical path leading from the light source via the object surface to the detecting surface for adjusting the intensity ration between the two mutally orthogonal polarized light components of the light beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for detecting the level of anobject surface, and in particular to an apparatus useful for making thesurface of a photosensitive substrate coincident with the focal plane ofa projection optical system, for example, in precise photography

2. Related Background Art

In a projection exposure apparatus for the manufacture of semiconductordevices, as a device for detecting the focus position of a projectionoptical system, a system whereby an incident light is obliquely appliedto a semiconductor wafer provided at a position whereat the image of amask pattern is formed by a projection lens and the level of the surfaceof the semiconductor wafer is detected on the basis of the reflectedlight obliquely reflected from the surface of the semiconductor wafer isdisclosed, for example, in Japanese Laid-Open Patent Application No.56-42205 and U.S. Pat. No. 4,650,983 which is an improvement thereof.

According to this known position detecting system, a light beam isobliquely projected onto an object surface to form a slit-like opticalimage on the object surface and the reflected light therefrom isre-imaged on a detector constructed of a photoelectric conversionelement. Correspondingly to a change in the level of the object surface,the position of the reflected optical image on the detector shifts onthe detector. By detecting the amount of this shift, it is possible todetermine whether the object surface is coincident with the focal planeof the projection lens.

However, when the surface position of a semiconductor wafer is to beactually detected by the use of the prior-art position detecting systemconstructed as described above, it has been found that there is acertain limit in the position detecting accuracy thereof. When the causeof it has been examined variously, it is often the case with the surfaceportion of a semiconductor wafer that a thin film such as photoresistadhered to a semiconductor substrate such as silicon, and when thethickness of the thin film has become the order of 1 to 2 μm,interference is caused between the reflected light reflected by thesurface of the thin film and the light transmitted through the surfaceof the thin film and reflected by the surface of the semiconductorsubstrate, and this is considered to cause a variation in thedistribution of light intensity in a direction perpendicular to theoptic axis of the reflected light. Incidentally, the light raytransmittance of a material formed of an organic substance, such asphotoresist, is generally relatively high for a wavelength longer thanthe sensitizing wavelength (e.g., red light), and this has led to theproblem that the reflected light from the front surface of thephotoresist layer and the reflected light from the back surface thereofare liable to interfere with each other, thus causing an error.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to realize, with theinfluence of the interference between reflected lights taken intoaccount, a surface displacement detecting apparatus whose detectionaccuracy can be improved more beyond the limit of the prior-artapparatus.

In the detecting apparatus of the present invention, polarizing opticalmeans for arbitrarily changing the intensities of a P-polarized lightcomponent and an S-polarized light component on a detecting surface isprovided at a predetermined position on an optical path leading from alight source to an object surface or on an optical path leading from theobject surface to said detecting surface.

The reflected light reflected by the object surface interferes inconformity with the thickness of a thin film on the object surface andmakes interference fringes on the detecting surface. The interferencefringe by the S-polarized light of this reflected light and theinterference fringe by the P-polarized light of said reflected light are180° out of phase with each other because the incidence angle varieswith Brewster's angle as the boundary. Accordingly, the variation in thelight intensity obtained with the coherent light of the S-polarizedlight component and the coherent light of the P-polarized lightcomponent which are inverted in phase with each other being combinedtogether is not proportional to the film thickness, but is greatlydisturbed. So, the polarizing optical means is provided at a suitableposition on the detection optical path to vary the ratio between theP-polarized light component and the S-polarized light component, andwhen the intensities of the two polarized light components are suitablyvaried by suitably rotatively adjusting the polarizing optical means bythe utilization of the phase difference of 180° between the P-polarizedlight component and the S-polarized light component, the apparent amountof surface deviation on the object surface becomes smaller and thus, thedetection error can be improved.

It is another object of the present invention to provide said polarizingoptical means, and more particularly a polarizing device which caneasily change the intensity ratio between two polarized light componentswhose directions of polarization are perpendicular to each other to anyvalue and can again combine the changed two polarized light componentsefficiently and emit them.

In one mode of the present invention, said polarizing device includesmeans for separating the incident light into two polarized lightcomponents whose directions of polarization are perpendicular to eachother, means for arbitrarily changing the intensity ratio between saidtwo separated polarized light components, and means for combining saidtwo polarized light components whose intensity ratio has been changedand forming a composite beam emerging in one direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the construction of an optical systemaccording to a first embodiment of the present invention.

FIG. 2 is an illustration showing the change in the position of a spotimage.

FIG. 3 illustrates a reflected light reflected by a thin film.

FIGS. 4A, 4B, 4C and 4D illustrate the movement of the centroid of thequantity of light.

FIG. 5 is a graph showing a detection error for the film thickness.

FIGS. 6A and 6B respectively illustrate the polarized conditions of thereflected light when the angle of incidence is smaller than Brewster'sangle and when the angle of incidence is greater than Brewster's angle.

FIGS. 7A and 7B are graphs respectively showing variations in the lightintensities of P-polarized light and S-polarized light for the filmthickness and a detection error for the film thickness.

FIG. 8 is a graph showing a variation in the light intensity and adetection error for the film thickness.

FIG. 9 is a perspective view showing a polarizing prism.

FIG. 10 is a graph of light ray transmittances.

FIG. 11 is a perspective view showing a polarizing plate.

FIG. 12 is a graph showing a variation in the light intensity and adetection error for the film thickness.

FIG. 13 schematically shows the construction of an optical systemaccording to a second embodiment of the present invention.

FIG. 14 is an optical arrangement diagram showing the construction ofthe correcting optical system of FIG. 13.

FIGS. 15, 16. 17 and 21 are optical arrangement diagrams showingmodifications of the correcting optical system.

FIGS. 18 and 19 are plan views showing the filters 130 and 131,respectively, of FIG. 17.

FIG. 20 shows the distribution of the emergent light beam from theoptical system of FIG. 17.

FIG. 22 illustrates the polarized condition.

FIGS. 23, 24, 25, 26, 27, 29, 30, 31 and 32 are optical arrangementdiagrams showing modifications of the correcting optical system.

FIG. 28 illustrates the action of the half-wave plate of FIG. 23.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the optical system of an oblique incidence type surfacelevel detecting apparatus. The paths of light rays indicated by solidlines show the conjugate relation of a slit image, and the paths oflight rays indicated by broken lines show the conjugate relation of alight source image.

A detecting light from a light source 1 emitting a so-calledrandom-polarized light which does not have a particular direction ofpolarization, such as a light-emitting diode (LED) or a halogen lamp,illuminates a slit plate 3 through a field lens 2. The slit plate has aslit 31 which is long in a direction perpendicular to the plane of thedrawing sheet, and a light beam L₀ projected through the slit 31 iscondensed by an objective lens 41 and a slit image is formed on thesurface 51 of a wafer 5. A reflected light L₁ reflected from the surface51 is condensed by an objective lens and a slit image is again formed ona slit plate 6. Also, the reflected light L₁ passed through a slit 61formed in the slit plate 6 is condensed as a detecting light L₃ on alight-receiving element 8 such as a photoelectric conversion element bya collector lens 7. The slit plate 6, the collector lens 7 and thelight-receiving element 8 together constitute a position detector.

The lengthwise direction of the slit 61, like that of the slit 31, isset to a direction perpendicular to the plane of the drawing sheet. Theslit plate 6 vibrates in a direction indicated by arrow a. Thus, theslit image re-formed on the slit plate 6 is scanned by the slit 61, andthe displacement of the wafer surface from the reference plane (thefocal plane) may be determined from the amount of deviation of the slitfrom the reference position when the detection signal from thelight-receiving element 8 becomes maximum.

A correcting optical system 9 is disposed on the optical path betweenthe objective lens 42 and the slit plate 6. This correcting opticalsystem 9 will be described later in detail. FIG. 2 shows the amount ofdisplacement of the slit image on the slit plate 6 when the wafersurface 51 is displaced along the optic axis Z of a projection lens 20.When the incident light L₀ is incident on the surface 51 lying at areference height Z₀ at an angle of incidence θ, a slit image formed at apoint Q₀ is re-formed at a reference position P₀ on the slit plate 6 bythe objective lens 42. When the surface 51 is displaced by ΔZ to aposition 52 in a direction Z, the incident light L₀ is reflected at apoint Q₁ and a reflected principal ray L_(S) reaches a point P₁ on theslit plate 6 through the objective lens 42, and a slit image isre-formed thereat. In this case, if the amount of displacement of theslit image from the reference position P₀ on the slit plate 6 to thepoint P₁ is Δy and the imaging magnification of the objective lens 42 isβ, the amount of displacement ΔZ of the surface 51 is given by

    ΔZ=Δy/(2β sin θ)                    (1)

On the other hand, where the object surface, as shown in FIG. 3, isconstructed of the surface 55 of a thin film 54 formed, for example, ofphotoresist applied onto a semiconductor substrate 53, part of a lightbeam L₀ incident at a point Q₀ on the surface 55 is not only reflectedas a reflected light L11, but also is transmitted through the thin film54 and reflected by the surface 56 of the substrate 53, and istransmitted through the surface 55 and emerges as a second reflectedlight L12. Thereafter, in a similar manner, third, fourth, . . .reflected lights L13, L14, . . . are produced, and these are consideredto be combined with the first reflected light L11 and reach the slitplate 6.

Examining this combined reflected light, the second reflected light L12once reflected in the interior of the thin film 54 can be considered tohave been reflected at a position apparently as deep as a distance δfrom the surface 55 and thus, on the slit plate 6, it is imaged whilelaterally deviating by an amount of deviation e represented by

    ε=2·β·sin θ·δ(2)

with the incident position P₀ of the regular reflected light L11 as thereference. Here, the apparent amount of deviation δ of the surface 55can be found as ##EQU1## In equation (3), d is the thickness of the thinfilm 54 and n is the refractive index of the thin film 54. Reflectedlights obtained by being reflected two times, three times, . . . , mtimes in the interior of the thin film 54 likewise deviate in positionby 2ε, 3ε, . . . , mε.

These reflected lights interfere with one another on the basis of theconditions of the optical system and the thickness d of the thin film.As a result, the shape of the image formed on the slit plate 6 isdeformed with a result that the centroid of the quantity of lightdetected by the position detector deviates, whereby an error occurs tothe result of the detection of the amount of deviation Δy based on theregular reflected light L11.

This phenomenon may be qualitatively examined as shown in FIGS. 4A-4D.

First, let it be assumed that when only the first reflected light L11has reached the slit plate 6, the position detector judges the positionof this centroid of the quantity of light as y₀ and with regard to thesecond reflected light L12 once reflected in the interior of the thinfilm 54, the position detector judges in accordance with equation (2)that the centroid of the quantity of light thereof is at a position y₀₁deviated by an amount of positional deviation ε. In this case, assumingthat as shown in FIG. 4A, the light intensity of the reflected light L11is a normalized value 2.0, the light intensity of the second reflectedlight L12 is weaker than this value, say, approximately 0.5.

Now, if the thickness d of the thin film 54 is sufficiently great andthe light beam L₀ from the light source 1 is of low coherence, thereoccurs no interference between the first reflected light L11 and thesecond reflected light L12. Accordingly, the light intensity of the slitimage formed on the slit plate 6 presents a distribution of lightintensity represented by the sum of the distribution of light intensityof the first reflected light L11 and the distribution of light intensityof the second reflected light L12, as shown in FIG. 4B. As a result, thecentroid of the quantity of light of the distribution of light intensityof the slit image formed on the slit plate 6 appears at a position y₁deviated by a slight amount of deviation Δy₁ relative to the centroid y₀of the distribution of light intensity of the reflected light L11.However, the amount of deviation Δy₁ varies in proportion to thethickness d of the thin film.

Actually, however, the thickness of the thin film d is as small as theorder of 1-2 μm and therefore, the reflected light therefrom is high inpotentiality of interference and in many cases, the reflected lights L11and L12 interfere with each other and these two lights strengthen eachother or weaken each other. Thus, the phenomenon that the shape of thecombined image formed on the slit plate 6 is destroyed occurs, wherebythe centroid of the quantity of light of the combined image deviatesgreatly from the centroid y₀ of the quantity of light.

For example, when the interference light intensity of the reflectedlight L12 relative to the reflected light L11 has become maximum, asshown in FIG. 4C, the light intensity of the interference portionL11+L12 becomes extremely great (in the case of the present embodiment,the light intensity of the reflected light is 4.5). The result is thatthe centroid of the quantity of light of the image moves to a positiony₂ deviated by a greater amount of deviation Δy₂ than in the case ofFIG. 4B.

In contrast, when the interference light intensity is minimum, as shownin FIG. 4D, the reflected lights L12 and L11 negate each other within arange over which they overlap each other, with a result that thecentroid of the quantity of light of the combined image moves by anamount of positional deviation Δy₃ extremely great as compared with thecentroid y₀ of the quantity of light and moreover to a position y₃deviated to the opposite side from FIG. 4C. Particularly in the case ofFIG. 4D, the amount of positional deviation Δy₃ is great, and this is afactor which causes a very great error in the determination of theposition.

FIG. 5 graphically shows the influence the interference of the reflectedlights as described above imparts to the detected position, and thehorizontal axis represents the thickness d of the thin film and thevertical axis represents the detection error of the position detectorwith the surface 55 of the thin film as the reference. A straight lineK3 indicated by phantom line is indicative of the position of the uppersurface 56 of the semiconductor substrate 53.

Assuming that the reflected lights do not interfere with each other, thelight intensities of the reflected lights from the surface 55 of thethin film and the surface 56 of the substrate are determined by thereflectances on the reflecting surfaces and constant. However, thereflected lights L12, L13, . . . from the surface 56 of the substratedeviate relative to the reflected light L11 in proportion to thethickness d of the thin film. Thus, the position of the centroid of thequantity of light of the slit image deviates from the detectionreference position P₀ in proportion to the thickness d of the thin film.Accordingly, the position detector indicates a linear detection errorproportional to the film thickness d as shown by solid line K1 with thelevel Z₀ of the surface 55 of the thin film as the reference.

However, when the interference phenomenon occurs as previouslydescribed, there is caused deviation not proportional to the filmthickness d which waves greatly along the solid line K under theinfluence of the interference, as indicated by a curve (broken line) K2.Particularly, as described with respect to FIG. 4D, sharply pointedthorn-like extreme deviation occurs in the vicinity of the filmthickness for which the reflected lights negate each other. Under such asituation assuming that for example, the thickness d of the thin film 54is irregular within a range from W₁ to W₂ (W₁ -W₂ =ΔW) in themanufacturing process as shown in FIG. 5, where the reflected light areincoherent (solid line K1), the result of the detection is onlyirregular by ΔX₁, but in the case of a curve K2 which indicates that thereflected lights interfere with each other, the result of the detectionbecomes greatly irregular within the maximum range of ΔX₂.

Now, in the description of the influence of the interference in FIG. 4,only the first reflected light L11 and the second reflected light L12have been qualitatively described, but actually, the reflection takesplace infinite times as shown in FIG. 3 and is very complicated.However, as compared with the first reflected light L11 and the secondreflected light L12, the light intensities of the other reflected lightsare weak and therefore, no great error will occur even if the influenceof the interference is typified by only said reflected lights L11 andL12.

Generally, a reflected light includes therein P-polarized light parallelto the incidence surface (perpendicular to the object surface) andS-polarized light perpendicular to the incidence surface, and withregard to the amplitude and phase of the light, the following Fresnel'sequations are established. When the angle of incidence is θi and theangle of refraction is θt and the amplitude of P-polarized light isR_(P) and the amplitude of S-polarized light is R_(S),

    R.sub.P =-tan(θi-θt)/tan(θi+θt)    (4)

    R.sub.S =-sin(θi-θt)/sin(θi+θt) (5)

Here, when the angle of incidence is equal to Brewster's angle, that is,when θi+θt=90°, tan(θi+θt)=∞ and sin(θi+θt)=1. Accordingly, theamplitude R_(S) of S-polarized light becomes a value conforming to therefractive index of the thin film, while the amplitude R_(P) ofP-polarized light becomes zero and thus, the P-polarized light is alltransmitted through the object surface. Also, if θi+θt<90° (or >90°),0<sin(θi+θt)<1 and accordingly, the sign of the amplitude R_(S) ofS-polarized light in equation (5) does not change, while tan(θi+θt)>0(or <0) and accordingly, the sign of the amplitude R_(P) of P-polarizedlight in equation (4) is reversed.

When the angle of incidence θi of the light beam L₀ onto the surface 55is smaller than Brewster's angle (that is, θi+θt<90°), as shown in FIG.6A, P-polarized light (arrows P1 and P2 parallel to the plane of thedrawing sheet) and S-polarized light (arrows S1 and S2 perpendicular tothe plane of the drawing sheet) are both in the same direction withrespect to the reflected light L11 and the reflected light L12 and thereis no phase shift therebetween. However, when the angle of incidence θiis greater than Brewster's angle (θi+θt>90°), as shown in FIG. 6B,S-polarized light is unchanged, while the P-polarized light in thereflected light L11 is reversed by 180°, unlike the case of FIG. 6A.That is, as regards the P-polarized light in the reflected light L11,the angle of incidence θi is reversed in phase with Brewster's angle asthe boundary.

With regard also the multiple times reflected lights L13, L14, . . .shown in FIG. 3, like the reflected light L12, it is considered thatP-polarized light is reversed by 180° relative to the reflected lightL11. The substrate 53 is usually formed of silicon, aluminum or thelike, and the reflected light from these substances also causes phasedeviation when the angle of incidence is great. However, since in thethin film (photoresist) 54, the angle of incidence θa is smaller thanBrewster's angle, little or no phase deviation is caused and as aresult, when the angle of incidence θi onto the surface 55 is greaterthan Brewster's angle, P-polarized light becomes about 180° out of phasealso with S-polarized light.

There will now be shown an example of the simulation of the coherentlight of P-polarized light and the coherent light of S-polarized lightin which even the multiple times reflected lights L13, L14, . . . aretaken into consideration. FIG. 7A shows variations in the intensities ofthe coherent lights for a variation in the thickness d of the thin film,and FIG. 7B shows a detection error for the actual surface position. Thesolid line indicates the curve by P-polarized light, and the broken lineindicates the curve by S-polarized light. In this case, it is assumedthat the light beam L₀ from the light source 1 is a monochromatic lightof wavelength λ740 μm and this light beam L₀ is projected onto thesurface of a semiconductor wafer comprising a silicon substrate (complexrefractive index n_(s) =3.71+0.01i), an aluminum layer (complexrefractive index n_(AL) =1.44+5.2i) attached to the surface of thesilicon substrate to a thickness of 1 μm, and photoresist (complexnumber n_(R) =1.64+0.002i) deposited thereon, at an angle of incidenceθ=70°, by the use of objective lenses 41 and 42 having a numericalaperture NA=0.1.

As is apparent from FIG. 7A, between P-polarized light (solid line) andS-polarized light (broken line), the period of the strength and weaknessof light by the interference effect deviates by approximately a halfperiod (180°) and when the light intensity thereof becomescorrespondingly weak, a position far below the photoresist (in which theamount of deviation from the surface is great) is detected as shown inFIG. 7B and thus, the detection error is great. For example, examiningthe intensities of the coherent lights and the detection error for thefilm thickness of 1.2 μm, P-polarized light (solid line) is maximum inintensity and moreover, the detection error is small and thus, the stateof FIG. 4C is assumed. Conversely, however, with regard to S-polarizedlight, the intensities of the coherent lights are in the vicinity ofminimum and the detection error is great and thus, the state of FIG. 4Dis assumed. So, when the two P-polarized light and S-polarized light arecombined together, it means that the distributions of quantity of lightof FIGS. 4C and 4D are applied coherently, and the centroid of thequantity of light thereof is drawn back toward the higher lightintensity (for example, in the direction of FIG. 4C).

FIG. 8 shows the result of the combination of P-polarized light andS-polarized light shown in FIGS. 7A and 7B. In FIG. 8, the broken linecurve indicates a variation in the intensity of the coherent light for avariation in the film thickness, and the solid line curve indicates adetection error for the actual surface position. As can be seen in FIG.8, the variation in the light intensity by the interference becomessmaller and the detection error Δ is still within the range of 0.32 μmfor the film thickness in the vicinity of 1.1 μm. However, if the ratiobetween the intensities of P-polarized light and S-polarized light issuitably changed by the use of means which will be described just below,it will be possible to minimize the detection error (the fluctuation ofthe detection error by a variation in the film thickness, indicated byΔX₂ in FIG. 5).

In order to improve the detection error of the surface position by theabove-described interference, a correcting optical system 9 is providedbetween the objective lens 42 and the slit plate 6, as shown in FIG. 1.This correcting optical system 9 is constituted by a polarizing prism 90as shown in FIG. 9. The reflecting surface 91 of the polarizing prism 90comprises a coating of dielectric multi-layer film applied to a cementedsurface inclined at 45°, and as shown in FIG. 10, the transmittanceT_(P) thereof for P-polarized light is approximately 100% and thetransmittance T_(S) thereof for S-polarized light is approximately 50%.Accordingly, the intensity ratio between the P-polarized light andS-polarized light of the light transmitted through this polarizing prism90 can be rendered into about 2:1. This ratio is very effective for thecorrection of the detection error in a case where the reflecting surface56 between the thin film 54 and the substrate 53 is formed by analuminum film.

Now, depending on the refractive index characteristics of the substrateand thin film, it is in some cases preferable to change the ratiobetween P-polarized light and S-polarized light to a value differingfrom said value. For example, by changing the characteristic of thereflecting surface 91, it is possible to change the ratio betweenP-polarized light and S-polarized light freely. Alternatively, byrotating this polarizing prism in the direction α about the optic axisof incidence, it is possible to change the ratio between P-polarizedlight and S-polarized light.

Also, the correcting optical system 9 may be replaced by a polarizingplate 92 as shown in FIG. 11. In such case, by rotating the polarizationaxis in the direction β, it is possible to change the ratio betweenP-polarized light and S-polarized light. That is, assuming that thepolarization axis X is rotated by β relative to the incidence surfacecontaining the optic axis of the light beam L₀ and perpendicular to theobject surface, the transmittance for P-polarized light is cos² β andthe transmittance for S-polarized light is sin² β, and by suitablyadjusting the angle β, it is possible to provide a desired ratio. Also,if design is made such that the transmittances of the polarizing prism90 shown in FIG. 9 are T_(P) =100% and T_(S) =0%, the polarizing prism90 can be used just in the same manner as the aforementioned polarizingplate 92.

FIG. 12 specifically shows the variations in the intensities of thecoherent lights and the detection error when, by rotatably providing thecorrecting optical system 9 on the detection optic axis shown in FIG. 1,P-polarized light and S-polarized light are combined together with theirratio as 1:0.35, under optical conditions similar to those in the caseof FIG. 7. As is apparent from FIG. 12, the detection error range Δ=0.21μm for the thickness of the thin film in the vicinity of 1.1 μm, andthis shows an improvement in accuracy as compared with Δ=0.32 μm shownin FIG. 8. That is, by relatively weakening S-polarized light, thedetection error by the S-polarized light is reduced to achieve animprovement in accuracy. It is also seen that the variations in theintensities of the coherent lights are narrowed in the width of strengthand weakness and thus improved, as compared with the variations in theintensities shown in FIG. 8.

Although in the above-described embodiment, the correcting opticalsystem 9 is provided on the reflection optical path on thelight-receiving side, it may be provided on the incidence optical pathon the light-emitting side, namely, between the light source and theobject surface, to obtain a similar correction effect. In that case, ifthe correcting optical system 9 is a polarizing prism, it is desirableto provide it on the portion of the optical path which is as near aspossible to the parallel light beam. However, if the opening angle ofthe light beam is small, the transmittances of the P-polarized lightcomponent and the S-polarized light component do not vary very much andtherefore, the location at which the correcting optical system isinstalled need not be specifically restricted.

Also, in the embodiment of FIG. 1, the light source used is one ofrandom polarization, but where a light source emitting a linearlypolarized light, for example, a semiconductor laser or a linearpolarization type laser is used as the light source 1, a rotatablehalf-wave plate may be used as the correcting optical system.Alternatively, the plane of polarization may be rotatively adjusted bythe utilization of a Faraday element whose plane of polarization can berotated about the optic axis by controlling a magnetic field, or anelement having optical activity of light (natural optical activity), forexample, a quartz plate. However, in the case of a quartz plate, asingle quartz plate of predetermined thickness or a combination of aplurality of such quartz plates is inserted in the optical path torotate the plane of polarization by a predetermined amount.

It is possible to incline the plane of polarization of the light sourcewith respect to the incidence surface by the use of the polarizingoptical means such as the half-wave plate, Faraday element or quartzplate to thereby change the relative ratio between the intensities ofthe P-polarized light component and the S polarized light component.

If a multi-wavelength light source is used as the light source 1 and thecoherence of the reflected light is reduced by polychromatic light, itwill be possible to further improve the detection accuracy. Also, designmay be made such that the centroid of the quantity of light is detectedby one of various detectors such as a CCD type solid state image pickupdevice, a PSD and an image pickup tube.

Description will now be made of a second embodiment of the presentinvention in which a correcting optical system is disposed in theincidence optical path between the light source and the object surface.

In FIG. 13, the light source 100 produces a non-polarized (randompolarized) or circularly polarized light beam. This light beam is madeinto a substantially parallel light beam L101 by a collimator lens 111and enters a correcting optical system 109, and emerges as a compositeemergent light beam L102 with the P-polarized light component and theS-polarized light component being changed into a suitable lightintensity ratio. This composite light beam L102 thereafter passesthrough a field lens 2 and a slit opening 31 in a slit 3 provided nearthe field lens, as in FIG. 1, and is projected onto the object surface51 through an objective lens 41 at an angle of incidence θi greater thanBrewster's angle.

The reflected light from the surface 51 is re-imaged on a slit plate 6by an objective lens 42.

FIG. 14 shows the specific construction of the correcting optical system109. A first polarizing prism 121 and a second polarizing prism 122 areconstructed similarly to those in FIG. 9. A light beam L101 entering thefirst polarizing prism 121 is separated by the joined surface thereofinto a P-polarized light component parallel to the incidence surface andan S-polarized light component perpendicular to the incidence surfaceand equal in intensity to the P-polarized light component, and theP-polarized light component is transmitted through the joined surfaceand travels toward the second polarizing prism 122. The S-polarizedlight component is reflected by the joined surface and travels via twomirrors 123 and 124 toward the second polarizing prism 122. TheP-polarized light is transmitted through the joined surface of thesecond polarizing prism 122, and the S-polarized light is reflected bythe joined surface, and the P-polarized light and the S-polarized lightare combined together and emerge as a composite light beam L102.

Density filters 125 and 126 whose transmittances can be changed asdesired are respectively provided on the two optical paths between thefirst polarizing prism and the second polarizing prism, and the bothdensity filters can have their transmittances changed from approximately100% to 0% as desired. Also, the incident light L101 is divided by thefirst polarizing prism into P-polarized light and S-polarized lightequal in intensity to each other without any loss of energy, and thedivided P-polarized light and S-polarized light are combined together bythe second polarizing prism without any loss of energy. Thus, where thetransmittances of the two density filters are set to 100%, the lightintensity of the composite light beam L102 becomes substantially equalto that of the incident light L101. In this case, assuming that theintensity of the incident light L101 is 100, the intensities of theP-polarized light and S-polarized light included in the composite lightbeam L102 are respectively 50, and such light intensities of theP-polarized light and S-polarized light can be freely changed between 50to 0 by the density filters 125 and 126, respectively, and the intensityratio between the P-polarized light and the S-polarized light can bechanged as desired to thereby provide the composite light beam L102.

By constructing the correcting optical system 109 as shown in FIGS. 15and 16, it is possible to make the optical path length of theP-polarized light and the optical path length of the S-polarized lightequal to each other. That is, the construction shown in FIG. 15 is suchthat the position of the second polarizing prism 122 is replaced withthe position of the mirror 124. Accordingly, in this case, the directionof travel of the beam L102 is orthogonal to the direction of travel ofthe beam L101.

In the construction of FIG. 16, the optical path lengths of theP-polarized light and the S-polarized light are equal to each other andthe beam L102 can be made to emerge in the same direction as the beamL101.

In FIG. 16, on the respective optical paths of the P-polarized light andthe S-polarized light which are equal in optical path length to eachother, there are provided half wave plates 127 and 128 adjacent thedensity filters 125 and 126, and the P-polarized light and theS-polarized light have their directions of polarization rotated through90° by these half-wave plates The light beam of the P-polarized lightcomponent is reflected by the joined surface of the second polarizingprism 122, and the light beam of the S-polarized light component istransmitted through the same joined surface, and the composite lightbeam L102 emerges in a direction parallel to the beam L101.

In FIGS. 14 to 16, the polarizing filter 92 shown in FIG. 11 may be usedinstead of the density filters 125 and 126. Also, in FIGS. 14 and 15,half-wave plates may be provided instead of the density filters 125 and126 and the polarized light components can be adjusted by suitablyrotating them. In this case, when the angle of rotation of the half-waveplates is θ, each polarized light component is

proportional to cos² (2θ).

The correcting optical system 109 may be constructed as shown in FIG.17, whereby there can be provided a composite light beam whose directionof polarization differs between the central portion and the marginalportion. On the optical path of the P-polarized light, as shown in FIG.18, there is provided a rotatable polarizing filter 130 having itscentral portion shielded from light, and on the optical path of theS-polarized light, as shown in FIG. 19, there is provided a rotatablepolarizing filter 131 having its marginal portion shielded from light.Also, these two polarizing filters 130 and 131 are disposed at suchpositions that the optical path lengths to the second polarizing prism122 are equal to each other.

Thus, the composite emergent light L102 emerging from the secondpolarizing prism has its central portion providing an S-polarized lightcomponent and has its marginal portion providing a P-polarized lightcomponent, as shown in FIG. 20, and by rotating the polarizing filters130 and 131 by a suitable angle, it becomes possible to change the ratiobetween the quantity of light of the marginal portion of the compositelight beam and the quantity of light of the central portion of thecomposite light beam as desired.

Where use is made of a light source which emits a linearly polarizedlight, the correcting optical system 109 can be constructed as shown inFIG. 21. That is, a quarter-wave plate 133 is provided on the opticalpath between a light source 200 which emits a linearly polarized lightand a first polarizing prism 121, whereby the linearly polarized lightbecomes a circularly polarized light, which enters the first polarizingprism 121. The incident light beam of this circularly polarized light,as in the case of FIGS. 14-17, is separated by the first polarizingprism 121 into a P-polarized light and an S-polarized light which areequal in intensity to each other, and these P-polarized light andS-polarized light are combined together by a second polarizing prism122.

In this case, the axis of the quarter-wave plate 133 may be fixed at 45°with respect to the directions of polarization of the P-polarized lightand the S-polarized light. If this is done, the intensity ratio betweenthe emerging P-polarized light component and S-polarized light componentwill be invariable as shown below even if the direction of polarizationof the linearly polarized light changes.

In FIG. 22, assuming that the linearly polarized light entering thequarter-wave plate 133 is a and the polarized lights emerging from thefirst polarizing prism 121 are P and S, the axis of the quarter-waveplate is inclined by 45° with respect to the directions P and S andtherefore, the light emerging from the quarter-wave plate 133 becomes anelliptically polarized light b whose axis of ellipse forms 45° withrespect to the directions P and S. Accordingly, the intensities of theP-polarized and S-polarized lights separated by the first polarizingprism 121 become equal to each other. Therefore, if the intensity ratiobetween the P-polarized light and the S-polarized light is adjusted bythe density filters 125 and 126, the intensity ratio between theP-polarized light and S-polarized light of the light beam L202 emergingfrom the second polarizing prism 122 will be invariable even if thedirection of polarization of the linearly polarized light of theincident light beam L201 changes.

The correcting optical system of FIG. 23 is designed to change theintensity ratio between the P-polarized light and the S-polarized lightby the use of a quarter-wave plate, and in this case, a light source 300emits a circularly polarized light. A quarter-wave plate 134 whoserotation is adjustable in a plane perpendicular to the optic axis of alight beam L301 is provided between the light source 300 and the firstpolarizing prism 121. The circularly polarized light is changed into alinearly polarized light by this quarter-wave plate, and the directionof polarization of the linearly polarized light can be inclined by anyangle with respect to a plane perpendicular to the joined surface of thefirst polarizing prism 121 by rotating the quarter-wave plate by asuitable angle of rotation. Thus, by suitably changing the angle ofinclination, the P-polarized light component and the S-polarized lightcomponent separated by the first polarizing prism 121 can be changedinto any intensity ratio. This construction is free of any loss of lightenergy and therefore, the intensity of the composite emergent light L302combined by and emerging from the second polarizing prism always assumesa value substantially equal to that of the incident light.

The correcting optical system of FIG. 24 is provided a light source 400emitting a linearly polarized light or an elliptically polarized lightand a half-wave plate 135. The mutually perpendicular x-directioncomponent and y-direction component of the light passed through thehalf-wave plate 135 are 180° out of phase with each other. Accordingly,by suitably adjusting the rotation of the half-wave plate 135 in a planeperpendicular to a light beam L401, it is possible to change theintensity ratio between the P-polarized light component and theS-polarized light component separated by the first polarizing prism.Where the light beam L401 is a linearly polarized light, the intensityratio can be freely changed between (0:100)--(50:50)--(100:0) as in FIG.23.

FIG. 25 shows an arrangement in which density filters 125 and 126 areadded to the construction of FIG. 24. If this is done, the intensitiesof the P-polarized light component and the S-polarized light componentcan be changed by both of the half-wave plate and the density filtersand therefore, with the loss of the energy of the incident light L401minimized, the intensity ratio between the P-polarized and S-polarizedlight components even of an elliptically polarized light can be changedas desired. Further, the light source may of course be one which emits acircularly polarized light or a random-polarized light, as well as onewhich emits any polarized light.

FIG. 26 shows an example of the correcting optical system using two (ormore) light sources of different polarized conditions.

Light beams from a first light source 100 which emits a random-polarizedlight or a circularly polarized light and a second light source 400which emits a linearly polarized light or an elliptically polarizedlight are changed over as desired by a change-over mirror 138 and entersthe first polarizing prism through a half-wave plate 135.

A modification of the correcting optical system shown below has lightmodulating optical means for modulating the P-polarized light componentand the S-polarized light component separated by the first polarizingprism 121, at a speed higher than the final time resolving power of thesubsequent detector. The P-polarized light and S-polarized light changedinto mutually incoherent lights by the light modulating optical meansare combined together and emerge. Thus, even if the plane ofpolarization of one of the P-polarized light and the S-polarized lightis rotated through 90° by the subsequent detecting optical system or thelike and overlaps the plane of polarization of the other, the mutualincoherence is maintained and the irregularity of imaging and thecreation of speckles caused by the interference of the P-polarized lightand S-polarized light can be prevented.

The correcting optical system of FIG. 27 is provided with a laser source200 which emits a linearly polarized light and a half-wave plate 140.This half-wave plate 140 is rapidly rotated about a light beam L201 byan actuator 142. The speed of rotation of the half-wave plate issufficiently higher than the time resolving power of the detector ofFIG. 13.

In FIG. 27, one polarized light component of the light beam L201 fromthe laser source 200 is E_(x), the other polarized light componentorthogonal thereto is E_(y), and the polarized light components of acomposite light beam L202 emerging from the second polarizing prism 122are E_(x2) and E_(y2). In this case, S-polarized light corresponds toE_(x) and P-polarized light corresponds to E_(y). In FIG. 28, there areshown the directions of polarization of the polarized light componentsE_(x) and E_(y) of the light beam L201 from the laser source 200. Itwill be understood from FIG. 28 that when the angular velocity of therotated half-wave plate 140 is a, the light beam L201 (E_(x), E_(y))becomes a light beam L204 expressed by E_(x1), E_(y1) of the followingequation (6), by passing through the half-wave plate.

In FIG. 28, the symbols ξ and η indicate the axis of the half-wave plate140, and it is to be understood that the light vibrating in thedirection η is delayed by λ/2 in phase with respect to the lightvibrating in the direction ξ. ##EQU2##

The light of the P-polarized light component E_(y1) passes through thefirst polarizing prism 121, the first density filter 125 and the secondpolarizing prism 122 in succession, and the then transmittance is α_(y)(α_(y) : complex number, and |α_(y) |≦1). Also, the light of theS-polarized light component E_(x1) passes through the first polarizingprism 121, the mirror 123, the second density filter 126, the mirror andthe second polarizing prism 122 in succession, and when the thentransmittance is α_(x) (α_(x) : complex number, and |α_(x) |≦1), thecomposite light beam L202 (E_(x2), E_(y2)) from the second polarizingprism 122 is represented by the following equation: ##EQU3## Fromequations (6) and (7), ##EQU4## Here, both α_(x) and α_(y) are complexnumbers, and the phase deviation resulting from the difference inoptical path length is included in these amounts of E_(x2) and E_(y2).The coherence matrix J₂ of the light beam L202 (E_(x2), E_(y2)) isrepresented by the following equation (9) if the time average is shownin < > ##EQU5##

When calculation is done with equation (8) substituted into equation(9), ##EQU6##

Here, the incident light L₀ is a linearly polarized light and therefore,E_(x) *E_(y) -E_(x) E_(y) *=0.

Accordingly, equation (10) becomes ##EQU7## and the diagonal componentbecomes zero.

This shows that the P-polarized light component (E_(y)) and S-polarizedlight component (E_(x)) of the composite light beam L202 do notinterfere with each other. Also, when the transmittances |α_(x) | and|α_(y) | are both 1, that is, when the density filters 125 and 126 areadjusted so as to transmit 100% of light therethrough, the emergentenergy of the light beam L202 is

    |E.sub.x |.sup.2 +|E.sub.y |.sup.2

and equals the energy of the light beam L201, and there is no loss ofenergy.

The mutual coherence of the P-polarized light component and S-polarizedlight component included in the composite light beam L202 has beendescribed above in detail with the aid of mathematical expression, andschematically it can be understood as follows. The light L204 passedthrough the half-wave plate 140 being rotated is a linearly polarizedlight, but the plane of polarization thereof rotates at a speed twice ashigh as the speed of rotation of the half-wave plate 140. This meansthat the light passed through the half-wave plate 140 is modulated andbecomes a random polarized light by the time average. Accordingly, therespective polarized light components separated by the first polarizingprism 121 subsequent to the half-wave plate 140 have no mutualcoherence, and the intensities thereof can be adjusted as desired by thedensity filters 125 and 126, and the composite light beam including theP-polarized light component and S-polarized light component combinedtogether at that adjusted intensity ratio and not interfering with eachother emerge from the second polarizing prism 122.

FIG. 29 shows a case where the laser beam emitted from the laser sourceis a circularly polarized light. A quarter-wave plate 144 and ahalf-wave plate 140 which is rotated are provided on the optical path ofthe circularly polarized laser beam L301 from the light source 300. Thebeam L301 is made into a linearly polarized light by the quarter-waveplate 144, and is made into a random polarized light L304 by the rotatedhalf-wave plate 140 and enters the first polarizing prism 121. Theincident light L1 is separated into P-polarized light and S-polarizedlight by the first polarizing prism 121. The process thereafter is thesame as that described with reference to FIG. 16. Rectangular prisms 223and 224 are functionally similar to the mirrors 123 and 124.

In FIG. 29, only a quarter-wave plate which is rotated can be usedinstead of the quarter-wave plate 144 and the half-wave plate 140 tomake the P-polarized light component and S-polarized light component ofthe composite light beam L302 incoherent with each other. In such case,the circularly polarized light from the light source 300, when passedthrough the quarter-wave plate, becomes a linearly polarized light, andwhen the quarter-wave plate is rotated, the direction of polarizationthereof is also rotated. Accordingly, it is equivalent to randompolarized light when the time average is taken into consideration.

Also, where the light beam from the light source is a linearly polarizedlight as in FIG. 27, a fixed quarter-wave plate may be used to firstmake the linearly polarized light into a circularly polarized light, andthis circularly polarized light beam may be made by a half-wave plateinto a linearly polarized light whose direction of polarization isrotated, and then may be caused to enter the first polarizing prism.

Furthermore, a usual elliptically polarized light beam can be changedinto a linearly polarized light by the use of a quarter-wave platefacing in a suitable direction and thereafter, P-polarized light andS-polarized light can be made incoherent with each other by a methodsimilar to that described in connection with FIG. 27 and moreover, theintensity ratio therebetween can be changed as desired. Also, when anelliptically polarized light whose axes of ellipse face in thedirections x and y passes directly through a rotating quarter-waveplate, the P-polarized light and S-polarized light separated by thepolarizing prism become incoherent, and the separated P-polarized lightand S-polarized light can be combined together with the intensity ratiotherebetween being variable, as in the above-described embodiment. Inthis case, however, the P-polarized light and S-polarized light formingthe light beam immediately after passed through the quarter-wave platediffer in intensity from each other and therefore, where a random lightin which P-polarized light and S-polarized light are equal in intensityto each other is finally required, at least one of the polarized lightscan be decreased by a filter.

If a phase modulating element or a Faraday polarizing element having amagneto-optical effect is used as the light modulating optical means,the present invention can also be applied to an apparatus of higher timeresolving power.

Referring to FIG. 30, instead of the rotated half-wave plate 140, aphase modulating element 150 is provided as the light modulating opticalmeans on the optical path of one of the P-polarized light andS-polarized light separated by the first polarizing prism. The linearlypolarized light beam L201 from the light source 200 passes through ahalf-wave plate 152 rotatively adjustable about the optic axis, and isseparated by the first polarizing prism 121. In the light beam of theseparated one polarized light (P-polarized light), there is provided aphase modulating element 150 using an electro-optical modulating elementsuch as, for example, KDP (potassium dihydrogen phosphate).

The phase variation caused by the phase modulating element 150 is madesufficiently faster than the time resolving power of the subsequentdetector or the like, whereby the P-polarized light component andS-polarized light component forming the composite emergent light L202become incoherent with each other.

Where a source of circularly polarized light is employed, a rotativelyadjustable quarter-wave plate may be provided instead of the half-waveplate 152, or a quarter-wave plate may be provided between the half-waveplate 152 and the light source.

Also, the phase modulating element 150 may be provided on the opticalpath of S-polarized light as indicated by phantom line. Furthermore,phase modulating elements may be provided on the optical paths,respectively, of P-polarized light and S-polarized light, but in suchcase, it is necessary to control the both phase modulating elements soas not to be turned with each other.

In FIG. 31, a phase modulating element 150 is provided rearwardly of thesecond polarizing prism 122 and a linearly polarized light beam L201passes through a quarter-wave plate 154 provided with its axis inclinedat 45° with respect to the directions P and S of the optical system.Accordingly, even if the direction of polarization of the incidentlinearly polarized light changes as described in connection with FIG.22, the intensity ratio between P-polarized light and S-polarized lightis unchanged.

Here, the axes of the phase modulating element 150 using electro-opticalcrystal in which double refraction occurs are made coincident with thedirections P and S and the phase variation thereof is effectedsufficiently rapidly, whereby the P-polarized light component andS-polarized light component forming the composite emergent light L202can be made incoherent with each other.

FIG. 32 shows an example in which the phase modulating element 150 isprovided forwardly of the first polarizing prism 121. A circularlypolarized light beam L301 is made into a linearly polarized lightforming an angle of 45° with respect to P-polarized and S-polarizedlights, by a quarter-wave plate 156, and enters the phase modulatingelement 150 such as electro-optical crystal with the optic axis thereofbeing coincident with the directions P and S. The phase modulatingelement 150 varies the phase difference between the direction P and thedirection S sufficiently rapidly. Therefore, the P-polarized lightcomponent and S-polarized light component of the light beam L304emerging from the phase modulating element 150 become equal in intensityto each other and incoherent with each other. The light beam L304 isseparated into P-polarized light and S-polarized light, and theseseparated polarized lights have their directions of polarization changedby polarizing plates or half-wave plates 157 and 158, and therefore theintensities of the P-direction component and S-direction component ofthe light beam L302 are changed. In this manner, there can be obtained alight beam in which the intensity ratio between mutually incoherentP-polarized light and S-polarized light has been changed into any value.

In each of the above-described embodiments, one of the polarizing prisms121 and 122 may be replaced by an amplitude dividing optical system suchas a half-mirror where the intensity of the composite light beam may bedecreased to half.

I claim:
 1. An apparatus for detecting the level of an object surface,comprising:means for supplying a light beam obliquely incident on saidobject surface; an imaging optical system provided to image said lightbeam reflected by said object surface; means having a detecting surfacecoincident with the imaging plane of said imaging optical system anddetermining the level of said object surface on the basis of theposition of the image of said light beam on said detecting surface; andpolarization correcting optical means provided on an optical pathleading from said beam supplying means via said object surface to saiddetecting surface for adjusting the intensity ratio between the twomutually orthogonal polarized light components of said light beam.
 2. Anapparatus according to claim 1, wherein said polarization correctingoptical means includes a polarizing prism.
 3. An apparatus according toclaim 1, wherein said polarization correcting optical means includes apolarizing plate.
 4. An apparatus according to claim 1, wherein saidpolarization correcting optical means includes separating optical meansfor separating the incident light into a pair of polarized lightcomponents whose directions of polarization are perpendicular to eachother, means for arbitrarily adjusting the intensity ratio between saidpair of polarized light components separated by said separating opticalmeans, and combining optical means for combining the pair of polarizedlight components whose intensity of ratio has been changed by saidadjusting means and for emitting them in one direction.
 5. An apparatusaccording to claim 4, wherein said separating optical means includesfirst polarizing prism means and said combining optical means includessecond polarizing prism means.
 6. An apparatus according to claim 5,wherein said arbitrarily adjusting means includes filter means providedin at least one of two optical paths of said pair of polarized lightcomponents between said first and second polarizing prim means.
 7. Anapparatus according to claim 5, wherein said arbitrarily adjusting meansinclude a wave plate member provided to be rotated about an optical axisof said incident light of said separating optical means.
 8. An apparatusaccording to claim 4, wherein said separating optical means includes afirst polarizing prism member having a surface functioning so that oneof said pair of polarized light components is transmitted therethroughand the other of said pair of polarized light components is reflected.9. An apparatus according to claim 4, wherein said polarizationcorrecting optical means further includes light modulating optical meansfor eliminating mutual coherence of said pair of polarized lightcomponents combined by said combining optical means.